A case often seen in shows like AFV and It Only Hurts When I Laugh, a ninja pole is a kind of pole of some sorts like a lamppost or street sign that you run into. You don't see it coming and you could've sworn it wasn't there, so when you walk along you ram into it and hilarity ensues.

I was walking down the street, minding my own business, then suddenly BAM! A fuckin' ninja pole got me!